This invention relates to yarns of brittle fibers protected by a thermoplastic shroud. These enshrouded yarns are useful in the preparation of preforms for making composite materials.
Advanced composite materials in which a matrix material is reinforced with fibers provide increased value-in-use over conventional materials. Metal matrix composites (MMC) exhibit high strength to weight ratios. Ceramic matrix composites (CMC) provide better property retention at elevated temperatures and/or in harsh environments or a combination of these desirable and enabling features. Plastic or resin matrix materials are also particularly useful in many situations.
The basic performance attributes of the matrix material are enhanced by the contributions of the reinforcing fiber(s). Especially useful are inorganic fibers based on aluminum oxide, silicon carbide or high modulus carbon fibers derived from pitch However, prior to infiltration of the fibers by the desired matrix material, a useful assembly of such reinforcing fibers, conventionally termed a preform, must be fabricated, usually in a shape near the net shape of the desired composite.
Preform preparation is especially difficult with such advanced reinforcing fibers as those identified above because, along with the desirable stiffness properties, there is an attendant difficulty in handling these fibers owing to their brittle nature. This is a well-recognized problem in the art and is addressed, for example, in McAliley et al. (U.S. Pat. No. 4,714,642), who propose a method of tow consolidation to permit ease of handling during the preparation of plastic or resin-matrix composites. Their process is enabled by use of polyacrylonitrile fibers, i.e., flexible textile-like materials, as the carbon fiber precursor, and the intended end-use is reinforcement of thermoplastic or thermosetting resins. Another avenue to handling brittle fibers is disclosed in Ganga (U.S. Pat. No. 4,614,678 and U.S. Pat. No. 4,713,139) wherein the intended matrix material, in particulate form, is incorporated in the fiber tow and contained there by a thermoplastic sheath. The resulting tow (or "roving") can be woven, braided, etc. then heated to form the desired composite. The sheath material may or may not be included in the resulting composite, depending on its melting point and decomposition characteristics, relative to the matrix material. A related development wherein the desired matrix is carbon is provided by Okura et al. (U.S. Pat. No. 4,772,502 and U.S. Pat. No. 4,902,453). The sleeve material enables further processing such as filament winding, weaving, etc. prior to converting by heat the precursor material (binder pitch, etc.) to provide carbon-carbon composites. Both Ganga and Okura et al. provide fibrous prepregs rather than more complex shapes and include matrix material or precursors thereof at the very beginning of their processes. The end result is not unlike melt coating, where the matrix is applied via the melt to the tow. Characteristically the result is monofilicity, where the tow acts as a rod rather than a yarn because the individual component filaments are not free to move relative to one another and therefore may break more easily than bend.
Reinforcement of resins or plastics (e.g., by prepregging) is much less demanding than reinforcement of metals, often infiltrated from the melt, or of ceramics, often done at high temperatures by chemical vapor infiltration. The stiffer, more costly and more brittle fibers used for MMC or CMC reinforcement--plus the requirement of preforming assemblies of such fibers to near net shape--imposes serious, additional performance and handling demands prior to infiltration. Fibers may be embedded in wax or resin (Dhingra; U.S. Pat. No. 4,869,339) to form sheets that can be laid up, etc., to form simple shapes. Filament winding to simple shapes is also possible. But the rigors of weaving, braiding and the like cannot be met by ceramic or carbon fiber yarns or tows in the unassisted state. Even in the as-produced state these tows have broken filaments that protrude, snag on guides, etc., and generally contribute to increased damage. Accordingly such tows may be wrapped with cellulosic (e.g., rayon) or thermoplastic yarns or fibers to assist in their processing. However, when such protective wrappers later are burned off, their shrinkage that attends decomposition will "cut" the reinforcement fiber and increase rather than decrease broken filaments. Twisting is not an option: additional damage is incurred, strength is diminished and interstitial volume available for infiltration by matrix material is decreased.
This invention protects brittle inorganic reinforcing fibers, specifically multifilamentary yarns and tows thereof, from damage due to processing, and allows sufficient interfilament mobility and permits reasonable bending without bundle breakage. Using the protected yarns of this invention useful woven, braided, etc. assemblies thereof can not only be prepared but such assemblies thereafter can be further handled and aggregated into useful three-dimensional preforms of near net shape, even where the desired shape is complex.